In radio communications systems, messages (for example voice, video information or other data) are transmitted by electromagnetic waves via a radio interface. The radio interface relates to a connection between a base station and subscriber stations, in which case the subscriber stations may be mobile stations or fixed-position radio stations. The electromagnetic waves are in this case transmitted at carrier frequencies which are in the frequency band intended for the respective system. For future radio communications systems, for example the UMTS (Universal Mobile Telecommunications System) or other 3rd generation systems, frequencies in the frequency band around 2000 MHz are envisaged.
A random access channel (RACH) in a radio communications system is distinguished since access to the channel is not coordinated. The mobile stations in a radio cell can use this channel without prior allocation in order, for example, to request subsequent allocation of radio resources, for example when setting up a connection.
However, uncoordinated access leads to collisions between the transmissions from individual mobile stations. When the transmissions from a number of mobile stations are superimposed at a receiving base station, the transmissions can no longer be detected. The mobile stations therefore do not receive any acknowledgement of the transmission.
After a collision, the mobile stations attempt to transmit via the random access channel. The more frequently access needs to be repeated, the longer the waiting time and the lower the level to which the efficiency of this access method falls.
DE 198 17 771 proposes that access blocks which are orthogonal with respect to one another in time be allowed, and that the probability of a collision be reduced by choosing one of a number of different access blocks. That is, different transmission times within the channel. A further option for improving the efficiency of the method has been disclosed in ETSI SMG2 UMTS L1 Expert Group, Tdoc SMG2 UMTS-L1 455/98, Oct. 14, 1998. This proposal is for the power level to be increased in steps. The mobile station starts at a transmission power level which is reduced to the normal power level setting, and increases the transmission power level in steps until the base station confirms reception.
These methods adjust the transmission power level purely deterministically so that the same collision problems occur repeatedly in the same scenario.
A method as claimed in the precharacterizing clause of claim 1 is described in WO 97 46041 A.